Spatial frequency channel models have been used to explain a broad range of phenomena in human vision involving the detection and apearance of spatial patterns. Contrast threshold experiments have shown narrow spatial frequency tuning, while masking experiments have shown broadband tuning. We have found that detection of two-frequency composites in the presence of a supra-threshold masking pattern reveals narrow tuning when the composites cause changes in apparent contrast and broad tuning when they change apparent form. Furthermore, changes of apparent form and changes of apparent contrast contribute independently to pattern discriminability. We will do a variety of experiments to fully explore the point of view that it is the character of supra-threshold pattern attributes, and the way spatial frequencies contribute to them, rather than the structure of a channelled input stage which underlies the phenomena observed in detection experiments.